Potassium is the third major plant and crop nutrient after nitrogen and phosphorus. It has been used since antiquity as a soil fertilizer (about 90% of its current use). It is mined throughout the world from potassium deposits, either in underground or surface mines, wherein potassium is found in different chemical forms, such as carbonate, chloride, sulphate and nitrate. Each of these chemical forms requires a different chemical procedure to extract and concentrate the potassium from the deposits.
Since potassium sulphate (K2SO4) does not contain chloride, it is the preferred choice for crops, which are sensible to chloride, and which include coffee and several fruits and vegetables. Also crops that are less sensitive to chloride may still require potassium sulphate for optimal growth if the soil accumulates chloride from irrigation water.
Various methods have been proposed so far regarding the production of potassium sulphate (also called “sulphate of potash” or SOP) and various routes have been explored. For example, solution mining of sylvinite (a mixture of KCl and NaCl) for the production of KCl is known and is a technology that is practiced, a.o. in cold climate areas, for example, in Saskatchewan, Canada.
U.S. Pat. No. 2,902,344 (SINCAT SPA, 1959) discloses a process for the recovery of potassium sulphate from kainite ore (KCl.MgSO4.H2O) containing sodium chloride as an impurity. The kainite ore is converted into schoenite by mixing with the mother liquor containing some potassium sulphate at 20° C., and further decomposed into SOP using warm water, preferably at about 45° C.
DE 1592035 A1 (SINCAT SPA, 1970) discloses a process for the recovery of potassium sulphate from kainite ore using a langbeinite (K2Mg2(SO4)3) suspension which is processed into schoenite (K2SO4.MgSO4.6H2O) and leonite (K2SO4.MgSO4.4H2O) at 20 to 35° C.
U.S. Pat. No. 3,058,806 (Metallgesellschaft, 1962) discloses a process for the production of SOP from kainite by the dissolution of kainite in hot water, which comprises a cooling step to form the schoenite crystals and reacting it with potassium chloride.
U.S. Pat. No. 3,589,871 (GREAT SALT LAKE MINERALS, 1971) discloses a method of producing kainite from natural brines containing potassium by adding MgCl2 and using evaporation in solar ponds to precipitate kainite and carnallite (KMgCl3.6(H2O)).
U.S. Pat. No. 3,634,041 (GREAT SALT LAKE MINERALS, 1972) discloses a process for the production of SOP from essentially pure schoenite.
WO 05/063626 A1 (Indian Council of Scientific Industrial Research, 2005) discloses a process for the production of SOP from bittern comprising a step wherein kainite is converted into schoenite, aqueous CaCl2) is used and crude carnallite is produced as an intermediate using a cooling step at ambient temperature (25° C.).
None of the known processes is able to operate entirely at temperatures above 35° C., using a minimum of water as well as electrical power, and can be operated with different potassium deposits or a mixture thereof.
The current invention discloses a method which could be operated at higher temperatures, in particular at temperatures above 35° C. and which does not require a process step operated at a temperature below 35° C., in particular a cooling step at 20 to 25° C. Although the use of the method according to the invention is not limited to said temperatures, the method according to the invention can be advantageously used in mining areas which are situated in warm or hot climates (such as the Dallol region in Ethiopia). Furthermore, the method of the invention is very energy-efficient as it does not use mechanical cooling, nor heating devices, and it uses low amounts of freshwater. Therefore, the method according to the invention is especially suitable for use in remote location where access to energy and auxiliary systems is difficult. Furthermore, the method according to the invention may start from a solution, obtained by solution mining such that different potassium salts and mixtures thereof can be processed. It is a further object of the process to minimize water usage, as well as to minimize power usage and cooling water. The process can be operated economically in a hot, dry area that has limited resources available.
The method according to the invention is based on the finding that schoenite does not form at temperatures above 35° C., more in particular above 40° C. and under the conditions of the described method, such that a method for the production of potassium sulphate was developed, based on the formation of leonite.